Respect the range in `to_stream_with_range` even when the body length is not yet known.
When a cache item is `Found`, the length of the cached item may or may not be known:
- the item may be fully cached;
- the item may be streaming in, but have a known length; or
- the item may be streaming in progressively, **without a known length**.
By default (legacy behavior), if a range is specified but the length is not known,
the contents of the entire item will be provided instead of the requested range.
`always_use_requested_range` indicates any cached item returned by this lookup should provide only the requested range,
regardless of whether the length is known. An invalid range will eventually return a read error, possibly after providing some data.
**NOTE:** In the future, the `always_use_requested_range` behavior will be the default, and this method will be removed.